Signal-processing devices, such as radar apparatuses, display an image where a target object is plotted with respect to the position of the device itself (or its antenna) on a display module so that a user can intuitively grasp the spatial relationship with the target object. Here, since pixels are typically arranged in a lattice structure (matrix) on the display module, when plotting the target object in the image, the pixel position of the target object is treated as a position in an XY rectangular coordinate system. Therefore, in the above signal-processing devices, for the indication on the display module, the pixel data in the XY rectangular coordinate system is typically generated. For example, a radar apparatus used for a ship or the like outputs a radio wave at a predetermined cycle from a rotating antenna (i.e., at every predetermined angle), sequentially plots data obtained from reflection waves of the radio waves to rectangular coordinates to visually express the spatial relationship between the ship and the target object which exists around the ship on the display module.
However, since the information acquired by the radar apparatus is distributed on a straight line which is radially drawn from the ship (located at the center) corresponding to a transmitting direction of the radio wave, a gap between adjacent straight lines becomes larger as a distance from the center becomes greater. Therefore, when the information acquired by the transmission and reception of the radio waves is simply displayed on the display module, an omission of the pixels is generated in a portion distant from the center position and, thus, it causes spoiling of the appearance of the display image. Therefore, in such a radar apparatus, the pixel omitted portion may be detected and interpolated. Such kind of radar apparatus is disclosed in JP11-352211A.
JP11-352211A indicates the radar apparatus configured as follows. The radar apparatus includes an image memory, a LAST detecting module, a pixel omission detecting module, and an interpolating module. The image memory converts coordinates of the reception data from polar coordinates to rectangular coordinates, and stores the converted coordinates. The LAST detecting module determines a matching on the basis of each of corresponding pixels between an arbitrary sample point i on previous sweep line data θn-1, and a plurality of proximity sample points on the previous sweep line data θn−1 and current sweep line data θn. Then, the LAST detecting module detects the sample point i as a LAST sample point when the corresponding pixel of the sample point i does not match with any of the corresponding pixels of the counterpart sample points. The pixel omission detecting module detects whether each corresponding pixel of the counterpart sample points matches with a pixel adjacent to the corresponding pixel of the LAST sample point on the sweep rotation side. Then, the pixel omission detecting module detects that, as the result of the above detection, there is a pixel-omitted state where the corresponding sample point does not exist in the adjacent pixel when none of those matches. The interpolating module interpolates the adjacent pixel as an interpolation pixel at the time of detecting the pixel omission with data of the adjacent pixel.
Because the pixel omitted portions may change variously in their generated positions and sizes according to parameters, such as the revolving speed of the antenna, the cycle at which the antenna outputs the radio waves, and a magnification of the image to be displayed, they may not be appropriately detected. Regarding this situation, the radar apparatus of JP 11-352211A can detect the omission of pixels adjacent to the pixel determined for LAST. However, when the omissions of pixels occur continuously in the rotating direction of the antenna (i.e., a sweep direction), the pixel omitted portions may not be detected.
In recent years, the display modules for the radar apparatus and the like have been able to perform more precise image expression because of the increase in the number of pixels. For this reason, the pixel omission which has not been expected in the conventional display screen having less number of pixels may newly occur in the recent display module. As a method of interpolating all of such pixel omitted portions, a method of interpolating pixel by pixel between adjacent straight lines (adjacent sweep lines) in the X-axis direction or the Y-axis direction may be considered. According to this method, the interpolation data can be drawn without leaving any pixel omitted portions.
In the meantime, in order to display a smooth radar image, it is necessary to generate the interpolation data so that the pixels drawn by the interpolation processing and the pixels drawn based on the sample data of the sweep lines are smooth therebetween without causing any visual uncomfortableness to the user. Regarding this point, in the method of interpolating pixel by pixel in the X-axis direction or the Y-axis direction, the direction of interpolating the pixels (X-axis direction or Y-axis direction) and the direction in which the continuity of the pixels should be considered (circumferential direction) are different. Therefore, it is difficult to generate the interpolation data simultaneously with the drawing of the interpolation data.